/Engine/Source/Runtime/Engine/Public/AnimationRuntime.h
The blend is done by taking a weighted sum of each atom, and re-normalizing the quaternion part at the end, not using SLERP.
static void BlendTwoPosesTogether ( const [FCompactPose](API\Runtime\Engine\FCompactPose) & SourcePose1, const [FCompactPose](API\Runtime\Engine\FCompactPose) & SourcePose2, const [FBlendedCurve](API\Runtime\Engine\Animation\FBlendedCurve) & SourceCurve1, const [FBlendedCurve](API\Runtime\Engine\Animation\FBlendedCurve) & SourceCurve2, const float WeightOfPose1, [FCompactPose](API\Runtime\Engine\FCompactPose) & ResultPose, [FBlendedCurve](API\Runtime\Engine\Animation\FBlendedCurve) & ResultCurve )